1. Field of the Invention
The invention relates to air purification systems, and more particularly, to a device for the purification of contaminated exhaust air through heterogenous catalysis.
2. Discussion of the Prior Art
A device for conducting catalytic reactions is proposed in German specification DE AS 1003192. The carrier material is a wire, preferably made of a chrome-nickel alloy, which is coated with a thin, firmly adhering coat of a temperature-stable and highly porous oxide, e.g., aluminum oxide, or with a mixture of various oxides. The coating is subsequently impregnated with an active catalytic material, e.g., platinum. After undergoing this treatment, the coated wires are wound into flat coils and mounted in a coil holder. In the resulting arrangement of multiple connected coils, the individual flat coils are insulated electrically and in terms of contact by an intervening mica layer. Disadvantageously, such a coil arrangement is instable at a particular transfer cross section. The flat coils must therefore be carefully held by their outermost edge in the coil holder, so as to avoid sag when a coil is heated. The total area available for heterogeneous catalysis on each flat coil is limited, since the outer edge is covered by the coil holder. In addition, the mica strip between the two flat coils forms a blocking layer, so that only part of the surface of the flat coil located above the mica layer (seen in the flow direction) is involved in the catalysis. Larger cross sections cannot be produced in the suggested manner, since without an appropriate support the flat coil would sag from its own weight. Skilled personnel are needed to manufacture and handle the flat coils, because the wires are very thin and have a diameter of less than one mm. Another disadvantage is that the resistance wire is coated with the oxide layer before being wound, so that there is always a danger that the brittle film coating will partially flake off, possibly leading to shorting contacts in the turn area.
A catalytic filter for diesel soot is proposed in published German specification DE 37 16 446 A1 in which a coating of metal-doped zeolite is applied to a permanent filter element, preferably a honeycomb body with reciprocally closed channels. The doped metal is preferably platinum. The suggested coating of the filter element serves to lower the ignition point for the thermal combustion of the adsorbed soot particles. In order to keep the required quantity of noble metal low, the noble metal is finely distributed by means of a zeolitic base. The effectiveness of this process must be questioned, since the dimensions of a soot particle are larger by a power of ten than the pore size of the zeolite used. This means that a large part of the applied noble metal cannot act catalytically, because the individual soot particles will not fit into the small pores and no contact is established with the doped noble metal particles located there. Furthermore, a large portion of the platinum is spatially fixed in the zeolite and cannot come into contact with the soot particles.
A generic device for the purification of contaminated exhaust air is known from DE 43 39 025 A1. This device consists of a non-metallic, gas-permeable and windable carrier element which utilizes individual ceramic fibers coated with a layer of a material that acts in an adsorptive manner. The carrier material is connected sandwich-fashion to a heatable metallic woven band, which band is coated with catalytic material. The entire element is located in a housing equipped with an inlet and an outlet through which the contaminated gas can be introduced and the purified gas can be extracted. A disadvantage of this arrangement is the excessively small specific surface of the carrier element and the unfavorable placement of the catalytic material.
A filter for diesel soot is disclosed in U.S. Pat. No. 5,180,409 which consists of a single-layer or multi-layer fabric of, preferably, ceramic yarn. For lower temperatures (between 330.degree. and 550.degree. C.) a glass fiber yarn can also be used. In order to reinforce the fabric, a flexible filling yarn is also woven into it. The fabric is not coated, and because of the large gaps between the individual yarn elements it is not suitable for use as an adsorbate. Furthermore, the filter cannot be regenerated and must be changed after a preset maximum pressure loss is reached.
It is therefore an object of the present invention to provide a compact, easily fabricated device for the purification of contaminated exhaust air through heterogenous catalysis with prior adsorption and desorption, with which it is possible, in a regenerable fashion and without removing the filter element, to purify the gas contaminated with various constituents more efficiently and using less energy, compared to known units, even at fluctuating contaminant concentrations of less than 100 mg/cbm.